1. Field of the Invention
The present invention relates to a combustion state detecting apparatus for an internal combustion engine, which apparatus is designed for detecting the combustion state of an air-fuel mixture within a cylinder or cylinders of the engine by detecting an ion current making appearance upon combustion of the air-fuel mixture. More particularly, the invention is concerned with a combustion state detecting apparatus for the internal combustion engine, which apparatus is imparted with a function or facility for estimating the causes for non-generation or disappearance of a combustion signal based on the ion current.
2. Description of Related Art
For having better understanding of the concept underlying the present invention, background techniques thereof will first be reviewed in some detail.
FIG. 5A is a circuit diagram showing schematically an arrangement of a hitherto known or conventional combustion state detecting apparatus for an internal combustion engine (hereinafter also referred to simply as the engine), which apparatus is designed for detecting the combustion state within an engine cylinder or cylinders on the basis of an ion current produced upon combustion of air-fuel mixture. Referring to the figure, reference numeral 1 denotes an ignition coil which includes a primary winding 11 having a high-voltage end connected to a positive electrode of a power supply source 3 such as an onboard battery, while the low-voltage end of the primary winding 11 is connected to a collector electrode of a power transistor 2 which serves as a switching element for turning on/off a primary current flowing through the primary winding 11. The power transistor has an emitter electrode connected to the ground potential. On the other hand, a secondary winding 12 of the ignition coil 1 has a high-voltage end connected to an electrode of a spark plug 4 disposed within a cylinder of the engine while a low-voltage end of the secondary winding 12 is connected to a bias circuit 5 which is designed to apply a bias voltage of positive polarity to the spark plug 4 through a wiring conductor.
Further, referring to FIG. 5A, reference numeral 6 denotes an ion current detecting circuit which is designed for detecting by way of the bias circuit 5 an ion current making appearance upon combustion of the air-fuel mixture and flowing through an inter-electrode gap of the spark plug 4 to thereby convert the ion current into a voltage signal. More specifically, the ion current detecting circuit 6 is implemented in the form of a conventional current mirror circuit constituted by a series connection of transistors Q1a and Q1b and a transistor Q2 which are connected in parallel between a positive voltage source (i.e., voltage source of plus or positive polarity) +V and the ground potential. A resistor R1 is inserted between the ground potential and the collector of the transistor Q2. A current analogous to the ion current flows through the resistor R1 to undergo a voltage conversion, whereby an ion current detection voltage signal (hereinafter referred to as the ion current detection signal) X2 is produced.
The ion current detection signal X2 is supplied to a decision circuit 7 which is designed for shaping the ion current detection signal X2 detected by the ion current detecting circuit 6 into a pulse signal, which then undergoes a processing for deciding occurrence of combustion of the air-fuel mixture, as a result of which a pulse-like decision signal X3 is outputted from the decision circuit 7 to be supplied to an ECU (Electronic Control Unit) 10.
More specifically, for shaping the ion current detection signal X2 resulting from the voltage conversion mentioned above into a pulse signal, the decision circuit 7 includes a comparator circuit composed of a comparator CP1 for comparing the level of the ion current detection signal X2 with a reference voltage Vth4, an integrating circuit composed of a resistor R2 and a capacitor C2 for eliminating noise components N1 and N2 superposed on the pulse-like ion current outputted from the comparator CP1, and a delay circuit composed of a comparator CP2. Parenthetically, it should be mentioned that a pull-up resistor R3 connected to the output terminal of the comparator CP2 serves for pulling up the output voltage level thereof.
Next, referring to a signal waveform diagram shown in FIG. 5B, description will be made of operations of the conventional combustion state detecting apparatus in conjunction with normal combustion, a first type of non-combustion event (e.g. due to absence of fuel supply) and a second type of non-combustion event (e.g. due to failure of generation of high voltage for firing).
1. Normal combustion
Upon rising of the ignition signal X1 applied to the base of the power transistor 2 under the control of the ECU 10, the current flowing through the primary winding 11 of the ignition coil 1 is interrupted, as a result of which a high voltage E is induced in the secondary winding 12 of the ignition coil 1, whereby a spark discharge is caused to take place within the inter-electrode gap of the spark plug 4.
The ion current generated due to the combustion of the air-fuel mixture within the engine cylinder in which the spark discharge has taken place is inputted to the ion current detecting circuit 6 by way of the bias circuit 5 to undergo the current-to-voltage conversion in the current mirror circuit, whereby the ion current detection signal X2 is outputted from the ion current detecting circuit 6.
At this juncture, it should be mentioned that the ion current detection signal X2 outputted from the circuit 6 contains in addition to the intrinsic ion current component due to the combustion of the air-fuel mixture a noise component N1 produced upon rising of the ignition signal and the noise component N2 making appearance upon termination or extinction of the spark discharge. Accordingly, these noise components N1 and N2 have to be eliminated before outputting the decision signal X3 for deciding the combustion event on the basis of the ion current.
Thus, before eliminating the noise components N1 and N2 through the medium of the delay circuit, the ion current detection signal X2 is inputted to the comparator CP1 constituting the comparator circuit for comparing the levels of the signal components of the ion current detection signal X2 with the reference voltage Vth4. Since each of the noise components N1 and N2 is intrinsically in the form of a spike-like signal, these components are shaped into pulses each having an extremely short time duration.
Thus, even when the pulse-like noise components N1 and N2 are inputted to the CR integrating circuit constituting a part of the delay circuit to thereby raise the charge voltage of the capacitor C2 up to or beyond the reference voltage Vth5 preset at the comparator CP2, the pulse-like noise components Ni and N2 will assume low level before the charge voltage of the capacitor C2 reaches the reference voltage Vth5 because the time duration of the noise components is short of the CR time constant. Consequently, no decision signal will be outputted from the comparator CP2 in response to the noise components N1 and N2.
On the other hand, when the ion current component undergone the pulse-shaping operation is inputted to the integrating circuit, the capacitor C2 is charged to a level equal to or exceeding the reference voltage Vth5 after lapse of a predetermined time, because the time duration of the pulse-like ion current component is greater than the CR time constant, as a result of which the output of the comparator CP2 becomes high, whereby the decision signal X3 indicating that the combustion has taken place normally, i.e., normal combustion, is outputted.
At this juncture, the time taken for the charge voltage of the capacitor C2 to exceed the reference voltage Vth5 will be defined as the mask period only for convenience of description. Then, the noise components making appearance upon rising of the ion current and termination or extinction of the spark discharge, respectively, can be eliminated during the mask period.
2. Non-combustion event due to absence of fuel supply
When the fuel supply is absent, i.e., unless the air-fuel mixture is normally charged into the engine cylinder, the ion current due to the combustion of the air-fuel mixture can not naturally flow. Of course, upon rising of the ignition signal X1 as well as upon extinction of the spark discharge occurring within the intra-electrode gap of the spark plug 4, the noise components N1 and N2 make appearance, which would be outputted as the ion current detection signal X2. However, because the noise components N1 and N2 are eliminated by the delay circuit described previously in conjunction with the normal combustion, the decision signal X3 attributable to these noise components can not be generated.
3. Non-combustion event due to failure of generation of secondary voltage by the ignition coil
As can easily be appreciated from the foregoing description, unless the high-voltage is induced in the secondary winding of the ignition coil 1 due to e.g. breakage of the primary winding thereof, neither the noise component N1 due to application (rising) of the ignition signal X1 nor the noise component N2 upon extinction of the spark discharge can be produced. Consequently, the decision signal X3 is not outputted.
As will now be understood from the foregoing, with the conventional combustion state detecting apparatus of the structure described above, it is certainly possible to detect the combustion events, i.e., combustion and non-combustion of the air-fuel mixture within the cylinder(s) of the internal combustion engine. However, it is impossible to determine discriminatively or identify the cause for occurrence of the non-combustion event. In other words, additional detecting means or facilities have to be provided for specifying the cause for occurrence of the non-combustion event, which will however incur increase of manufacturing cost and overhead inclusive of processing time, giving rise to problems.